This invention generally relates to techniques for testing and detecting the characteristics of a communication channel. More specifically, it relates to a method and system for detecting digital end-to-end transmission links.
A computer user may access a computer network from a remote location by establishing a modem connection over a communication network such as the public switched telephone network (xe2x80x9cPSTNxe2x80x9d) or other private networks. The computer modem uses the network connection to allow the remote computer user to communicate with the network using a communication protocol such as the ITU-T defined V.34 or V.90 protocol standards. The V.34, V.90, and most other modem communication protocols are designed to allow digital computer information to be transmitted over an analog communication channel.
Today, an increasing number of digital network connections such as ISDN or T1/E1 are becoming more widely available. In fact, digital end-to-end connections from a caller to a computer network may soon become common place. Computer modems, however, have traditionally been designed to allow digital computers to communicate with a computer network over analog communication channels by modulating the digital signals into a format suitable for transmission over the analog channel. With the advent of all-digital end-to-end communication links, computer modems may also include a digital communication mode that eliminates the conversion of digital signals to a modulation suitable for transmission over an analog communication link and the resulting re-conversion of the analog modulation back to digital signal by the receiver.
A problem with implementing a modem having a digital communication mode is detecting when the modem has a completely digital end-to-end communication link and is thus suitable for communication using a digital mode. To use the digital mode, the modem must know whether it is communicating over a digital end-to-end communication link or whether the transmission channel may have an intervening analog connection.
Methods for detecting digital end-to-end communication links have been proposed. For example, a method has been proposed that simply assumes the transmission channel includes only digital links and begins a line probing sequence over the connection as if it were a completely digital connection. The line probing sequence involves sending a sequence of pulse code modulation (xe2x80x9cPCMxe2x80x9d) codes over the communication link. The response of the transmission channel to these PCM codes can be used to determine if the connection is formed entirely by digital communication links. For example, the sequence of PCM codes may contain a large component at the 4 KHz frequency that may not be received because an intervening analog link is present. If the digital line probing sequence fails, the modem initiates an analog communication protocol. A further description of such a line probing sequence is discussed in xe2x80x9cProposed Startup Sequence for the v.pcm All-Digital Modexe2x80x9d ITU-Telecommunication Standards Sector publication PCM""97-174, Study Group 16xe2x80x94Question 23, by Jim Renkel.
One problem with the proposed digital mode is that if the connection includes analog or adaptive differential PCM links, the digital mode fails and the analog communication protocol must then be initiated. Like analog lines, ADPCM lines are also of insufficient quality for carrying digital communications. A substantial amount of connection time is wasted in attempting to set up digital communications and failing and a further delay is then incurred in setting up the analog communication protocol. While there are other methods of characterizing a transmission channel they may not work properly with voice grade links which may have echo cancellers and digital attenuation pads. Thus, a method and system is needed for detecting completely digital end-to-end communication links.
In accordance with an illustrative embodiment of the present invention, problems associated with the detection of end-to-end digital transmission channels are addressed. The present embodiment provides a method and device for detecting the presence of end-to-end digital communication links that are free of any intervening analog or ADPCM connection links.
In an illustrative embodiment, a method of detecting an end-to-end digital connection is described. The method includes the steps of transmitting a signal with a D.C. offset value across the communication channel, receiving the transmitted signal from the communication channel and detecting the D.C. offset value in the received signal. If the D.C. offset value is detected, the communication channel is determined to be a digital end-to-end connection. If the D.C. offset value is not detected in the received signal, the transmission channel includes analog or ADPCM connection links and is not suitable for digital communications.
In another embodiment of the invention, the described method includes the steps of adding an offset value to a signal and transmitting a transmitted signal comprising the offset value and the signal. Preferably, the offset value is a constant value. The method may also include the steps of receiving the transmitted signal as a received signal and detecting the offset value in the received signal. The detection of the offset value in the received signal indicates an end-to-end digital communication link. The absence of the offset value in the received signal indicates the presence of an analog transmission link in the connection.
In another aspect of the invention, an illustrative embodiment of a communication device for detecting digital connections is described. The device includes an adder for adding an offset value to a signal to form a transmit signal and a transmitter for transmitting the transmit signal across a transmission channel to a receiving communication device. The transmission channel may include a digital end-to-end connection or a connection that includes both digital and analog connection links. If the transmission channel includes only digital communication links, the added offset value will be present in the signal received at the receiving communication device.
Alternatively, in another aspect of the invention, an embodiment of the device includes a transmitter capable of transmitting an offset transmit signal, a receiver capable of receiving the transmitted signal and logic capable of detecting an offset signal. The logic may include an integrator to receive the transmitted signal, an absolute value circuit to determine the absolute value of the integrator output, and a comparator to compare the absolute value of the integrator output to a threshold value. If the absolute value of the transmitted signal is greater than a threshold value, the offset is detected indicating the presence of a digital end-to-end transmission channel. If the absolute value of the integrator is less than a threshold value, the D.C. offset is not detected indicating the presence of an analog link in the transmission channel.
In an embodiment of the invention, executable software code and a computer system with memory is used to implement the described embodiment. Alternatively, dedicated hardware, discrete logic, programmable logic devices (xe2x80x9cPLDxe2x80x9d), application specific integrated circuits (xe2x80x9cASICxe2x80x9d) may be used to implement the described embodiment.
Using the described embodiments to determine the presence of digital end-to-end communication links provides several advantages. For example, the described embodiment allows modems to determine the characteristics of a transmission channel before initializing communications. The modems may detect the presence of a digital end-to-end transmission channel and establish a high-speed digital communication protocol over the channel. Transmission channels with analog links can also be detected prior to establishing communications over the link, therefore avoiding wasting time attempting to establish a digital communication protocol or falling back to an analog communication mode.
The foregoing and other features and advantages of an illustrative embodiment of the present invention will be more readily apparent from the following detailed description, which proceeds with references to the accompanying drawings.